This application is based on Japanese Patent Application No. 2002-280188 filed on Sep. 26, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a polarization beam splitter that separates P- and S-polarized light.
2. Description of the Prior Art
Polarization beam splitters that separate polarized light components that are polarized on mutually perpendicular polarization planes are used in optical systems of image display apparatuses and optical disk apparatuses. A polarization beam splitter is provided with a dielectric multilayer film having two types of dielectric with different refractive indices alternately laid on top of one another so that each layer has an optical film thickness equal to xc2xc of the wavelength of light to be separated. Of the light obliquely incident on this dielectric multilayer film, P-polarized light is transmitted therethrough and S-polarized light is reflected therefrom. This makes possible the separation of the two differently polarized light components.
For efficient separation of P- and S-polarized light, it is advisable to make the angle of incidence at which light is incident on the dielectric multilayer film as close to the Brewster angle as possible. Moreover, it is preferable that the refractive index of the dielectric multilayer film and the angle at which the light travels therethrough fulfill formula (0) below.
sin2xcfx86=NH2xc2x7NL2/[NE2xc2x7(NH2+NL2)]xe2x80x83xe2x80x83(0)
where NE represents the refractive index of the medium that is located contiguous with the dielectric multilayer film and from which light enters the dielectric multilayer film, NH represents the refractive index of the high-refractive-index layers of the dielectric multilayer film, NL represents the refractive index of the low-refractive-index layers of the dielectric multilayer film, and xcfx86 represents the angle of the light relative to a normal to the dielectric multilayer film.
Moreover, it is necessary to protect the dielectric multilayer film and to package it into an easy-to-handle optical device.
In consideration of these requirements, a polarization beam splitter is generally composed of a dielectric multilayer film sandwiched between two transparent media, and the dielectric multilayer film is designed to be used at an angle of incidence of 45xc2x0 and to fulfill formula (0). Moreover, to minimize the deflection of light at the interface between the transparent media and air, and to make easy the handling of separated light by making P- and S-polarized light components travel in mutually perpendicular directions after separation, the two transparent media are each formed into the shape of a prism of which the section has the shape of a right-angled isosceles triangle, with the dielectric multilayer film sandwiched between the hypotenuse surfaces of those prisms.
A conventional polarization beam splitter built as described above exhibits, when light is incident on the dielectric multilayer film at an angle of incidence of 45xc2x0, high transmissivity for P-polarized light and high reflectivity for S-polarized light over a wide wavelength range about a reference wavelength, and thus it separates the two differently polarized light components very effectively. However, a conventional polarization beam splitter exhibits high dependency on angle of incidence; that is, when the angle of incidence at which light is incident on the dielectric multilayer film deviates even slightly from the design value of 45xc2x0, the polarization beam splitter separates P- and S-polarized light less effectively, exhibiting particularly lower transmissivity for P-polarized light. That is, unless light is incident at 45xc2x0, the wavelength range in which differently polarized light components are separated effectively is extremely narrow.
Some optical systems employing a polarization beam splitter handle only light of a single wavelength. However, many such optical systems handle light spreading over a certain range of wavelengths. For example, optical systems for use in image display apparatuses handle light spreading all over the range of wavelengths of visible light. To miniaturize such optical systems, it is preferable to use as few polarization beam splitters as possible, and to make a convergent or divergent beam of light incident on a polarization beam splitter.
However, because of the above-mentioned dependency on angle of incidence, it is impossible to effectively separate light spreading over a wide wavelength range when the light is in the form of a convergent or divergent beam. Even when the principal ray of a convergent or divergent beam of light is made incident on a dielectric multilayer film at an angle of incidence of 45xc2x0 as designed, the rays other than the principal ray are incident at angles deviated from 45xc2x0. Thus, only the portion of the beam quite near its principal ray is separated effectively into P- and S-polarized light, and the other portion, particularly the peripheral portion, of the beam is separated markedly less effectively.
FIGS. 25 to 27 show the relationship between the wavelength of light and transmissivity in a conventional polarization beam splitter. In these figures, thick lines represent the transmissivity for S-polarized light, and thin lines represent the transmissivity for P-polarized light. FIG. 25 deals with the ray that makes an angle of 45xc2x0 with a normal to the dielectric multilayer film before entering the prism (i.e., in the layer of air). This ray is incident on the dielectric multilayer film at an angle of incidence of 45xc2x0. FIG. 26 deals with the ray that makes an angle of 34.7xc2x0 with a normal to the dielectric multilayer film before entering the prism. FIG. 27 deals with the ray that makes an angle of 55.3xc2x0 with a normal to the dielectric multilayer film before entering the prism. These three rays correspond to the principal ray and the two outermost rays of a convergent or divergent beam of light of which the f-number as observed in the layer of air is 2.8 and of which the principal ray makes an angle of 45xc2x0 with the dielectric multilayer film. The refractive index Nd of the prism is 1.84.
As FIG. 25 clearly shows, with the principal ray, which is incident on the dielectric multilayer film at an angle of incidence of 45xc2x0, it is possible to separate P- and S-polarized light effectively over a wide wavelength range of from about 440 nm to about 640 nm. However, as FIG. 26 shows, with the outermost ray that is incident on the dielectric multilayer film at the smallest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 490 nm to about 540 nm and from about 600 nm to about 670 nm. Moreover, as FIG. 27 shows, with the outermost ray that is incident on the dielectric multilayer film at the largest angle of incidence, effective polarization separation is possible only in wavelength ranges of from about 380 nm to about 430 nm and from about 490 nm to about 600 nm.
Thus, the wavelength range in which the whole beam of light having an f-number of 2.8 is effectively separated into P- and S-polarized light is extremely narrow, namely from 490 nm to 540 nm. The greater the f-number of the beam of light, the wider the wavelength range in which effective polarization separation is possible, but increasing the f-number poses a demanding requirement on the beam of light to be subjected to polarization separation. This makes it difficult to miniaturize optical systems incorporating polarization beam splitters.
As one way to overcome this inconvenience, there have conventionally been proposed (for example, in Japanese Patent Applications Laid-Open Nos. H7-281024, H11-211916, and 2001-350024) polarization beam splitters in which the dielectric multilayer film is composed of two multilayer portions, of which the first is designed to fulfill formula (0) at the angle (xcfx861) of a first ray with respect to a first reference wavelength and of which the second is designed to fulfill formula (0) at the angle (xcfx862) of a second ray with respect to a second reference wavelength. In a polarization beam splitter built in this way, the wavelength range in which each multilayer portion allows effective polarization separation with low dependency on angle of incidence is no wider than that obtained conventionally, but it is possible to make the two multilayer portions cover different wavelength ranges so that the dielectric multilayer film as a whole allows effective polarization separation with low dependency on angle of incidence in a wider wavelength range.
However, simply designing a dielectric multilayer film to cope with different reference wavelengths and different angles of light does not always result in the two multilayer portions allowing effective polarization separation in continuous wavelength ranges. If the wavelength ranges in which the two multilayer portions allow effective polarization separation are not continuous, the dielectric multilayer film as a whole may allow effective polarization separation in a wider wavelength range, but this wavelength range includes, within itself, a partial wavelength range in which polarization separation is less effective.
An object of the present invention is to provide a polarization beam splitter that allows effective polarization separation in a wide wavelength range and that exhibits low dependency on angle of incidence all over that wavelength range.
To achieve the above object, according to the present invention, in a polarization beam splitter provided with a first prism including a first surface on which light is incident and a second surface that makes an acute angle with the first surface, a second prism disposed so as to face the second surface of the first prism, and a dielectric multilayer film sandwiched between the surfaces of the first and second prisms that face each other and composed of a first multilayer portion having high-refractive-index layers and low-refractive-index layers laid alternately on one another so that each layer has an optical film thickness equal to xc2xc of a first wavelength and a second multilayer portion having high-refractive-index layers and low-refractive-index layers laid alternately on one another so that each layer has an optical film thickness equal to xc2xc of a second wavelength, the condition expressed by formula (1) is fulfilled, and in addition, for angles xcex81 and xcex82 that respectively fulfill the conditions expressed by formulae (2) and (3), the conditions expressed by formulae (4) and (5), or (6) and (7), are fulfilled.
xcex1 less than xcex2xe2x89xa61.55xc2x7xcex1xe2x80x83xe2x80x83(1)
sin2xcex81=NH12xc2x7NL12/[Nd2xc2x7(NH12+NL12)]xe2x80x83xe2x80x83(2)
xe2x80x83sin2xcex82=NH22xc2x7NL22/[Nd2xc2x7(NH22+NL22)]xe2x80x83xe2x80x83(3)
|NH1xe2x88x92NL1| less than |NH2xe2x88x92NL2|xe2x80x83xe2x80x83(4)
|xcex81xe2x88x92xcex8| less than |xcex82xe2x88x92xcex8|xe2x80x83xe2x80x83(5)
|NH1xe2x88x92NL1| greater than |NH2xe2x88x92NL2|xe2x80x83xe2x80x83(6)
|xcex81xe2x88x92xcex8| greater than |xcex82xe2x88x92xcex8|xe2x80x83xe2x80x83(7)
where xcex1 represents the first wavelength, xcex2 represents the second wavelength, xcex8 represents the angle that the first and second surfaces of the first prism make with each other, Nd represents the refractive index of the first prism, NH1 represents the refractive index of the high-refractive-index layers of the first multilayer portion, NL1 represents the refractive index of the low-refractive-index layers of the first multilayer portion, NH2 represents the refractive index of the high-refractive-index layers of the second multilayer portion, and NL2 represents the refractive index of the low-refractive-index layers of the second multilayer portion. The angles xcex81 and xcex82 are those formed by the light traveling through the dielectric multilayer film relative to a normal thereto.
In this polarization beam splitter, light is introduced into it through the first surface of the first prism, and is then separated into P- and S-polarized light by the dielectric multilayer film sandwiched between the first and second prisms. The dielectric multilayer film is composed of the first and second multilayer portions, of which the first is designed to fulfill formula (0) noted earlier at a first angle xcex81 with respect to a reference wavelength set at a first wavelength xcex1 (formula (2)) and of which the second is designed to fulfill formula (0) at a second angle xcex82 with respect to a reference wavelength set at a second wavelength xcex2 (formula (3)).
Here, the second wavelength xcex2 is longer than the first wavelength xcex1, and the second wavelength xcex2 is so set as to be equal to or shorter than 1.55 times the first wavelength xcex1. The angle xcex8 that the first and second surfaces of the first prism makes with each other (i.e., the prism vertex angle) is equal to the angle of incidence at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film. Thus, between the first angle xcex81, which fulfills formula (2) in the first multilayer portion, and the angle of incidence xcex8 at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film, there is a difference of which the absolute value is equal to |xcex81xe2x88x921|. Likewise, between the second angle xcex82, which fulfills formula (3) in the second multilayer portion, and the angle of incidence xcex8 at which the ray that is incident perpendicularly on the first surface is incident on the dielectric multilayer film, there is a difference of which the absolute value is equal to |xcex82xe2x88x921.
These differences in angle are determined according to which is greater of the difference between the refractive indices of the high-refractive-index and low-refractive-index layers in the first multilayer portion and that in the second multilayer portion. When the difference between the refractive indices of the high-refractive-index and low-refractive-index layers is greater in the second multilayer portion than in the first multilayer portion (formula (4)), the difference between the second angle and the prism vertex angle is made greater than the difference between the first angle and the prism vertex angle (formula (5)). On the other hand, when the difference between the refractive indices of the high-refractive-index and low-refractive-index layers is greater in the first multilayer portion than in the second multilayer portion (formula (6)), the difference between the first angle and the prism vertex angle is made greater than the difference between the second angle and the prism vertex angle (formula (7)).
When formula (1) is fulfilled, and in addition, for the angles xcex81 and xcex82 that respectively fulfill formulae (2) and (3), formulae (4) and (5), or (6) and (7), are fulfilled, then the first and second multilayer portions allow effective polarization separation with low dependency on angle of incidence in different wavelength ranges, with an overlap secured between those two wavelength ranges. That is, it is possible to realize a dielectric multilayer film of which the dependency on angle of incidence remains low over two continuous wavelength ranges.
In one example, the high-refractive-index layers of the first multilayer portion is made of TiO2 or Ta2O5, the low-refractive-index layers of the first multilayer portion is made of MgF2 or SiO2, the high-refractive-index layers of the second multilayer portion is made of TiO2 or Ta2O5, and the low-refractive-index layers of the second multilayer portion is made of a mixture of Al2O3 and La2O3, a mixture of Al2O3 and La2O, or Al2O3.
In another example, the high-refractive-index layers of the first multilayer portion is made of a mixture of TiO2 and La2O3, a mixture of TiO2 and La2O, or Ta2O5, the low-refractive-index layers of the first multilayer portion is made of MgF2 or SiO2, the high-refractive-index layers of the second multilayer portion is made of TiO2 or Ta2O5, and the low-refractive-index layers of the second multilayer portion is made of a mixture of Al2O3 and La2O3, a mixture of Al2O3 and La2O, or Al2O3.
In another example, the high-refractive-index layers of the first multilayer portion is made of a mixture of Al2O3 and La2O3 or a mixture of Al2O3 and La2O, the low-refractive-index layers of the first multilayer portion is made of MgF2, the high-refractive-index layers of the second multilayer portion is made of TiO2 or Ta2O5, and the low-refractive-index layers of the second multilayer portion is made of Al2O3.
In another example, the high-refractive-index layers of the first multilayer portion is made of Al2O3, the low-refractive-index layers of the first multilayer portion is made of MgF2, the high-refractive-index layers of the second multilayer portion is made of TiO2 or Ta2O5, and the low-refractive-index layers of the second multilayer portion is made of SiO2.
The first and second prisms may each have the shape of a right-angled isosceles triangle.
The first and second prisms may have equal refractive indices for the same wavelength.
The ratio xcex2/xcex1 of the wavelength xcex2 to the wavelength xcex1 may be equal to or higher than 1.1.
The light introduced through the first surface may be visible light spreading over a predetermined wavelength range between 480 nm and 750 nm.
The light introduced through the first surface may be in the form of a convergent or divergent beam of which the f-number as observed in the layer of air is 2.8 and of which; the principal ray makes an angle of 45xc2x0 with the dielectric multilayer film.